The term “battery” originally meant a plurality of electrochemical cells connected in series in a housing. However, single electrochemical cells are nowadays frequently also referred to as batteries. During the discharge of an electrochemical cell, an energy-supplying chemical reaction which is made up of two electrically coupled but physically separate subreactions takes place. In an oxidation process, electrons are liberated at the negative electrode, resulting in flow of electrons via an external load to the positive electrode which takes up a corresponding quantity of electrons. A reduction process therefore takes place at the positive electrode. At the same time, an ion current corresponding to the electrode reaction occurs within the cell. This ion current is ensured by an ionically conductive electrolyte. In secondary cells and batteries, this discharge reaction is reversible. It is therefore possible to reverse the transformation of chemical energy into electrical energy which occurs during discharge. If the terms “anode” and “cathode” are used in this context, the electrodes are generally named according to their discharge function. In such cells, the negative electrode is therefore the anode, and the positive electrode is the cathode.
Similarly to secondary cells and batteries, capacitors can also store electrical energy in a reversible manner, however not in chemical form but rather in an electric field between two electrically conductive surfaces, the capacitor electrodes, which are usually arranged at a short distance from one another. Since electrical charge in a capacitor is not initially liberated or stored as a result of a chemical reaction, it can be taken up and released again very quickly. For this reason, the energy density of a capacitor is generally very clearly below that of a battery.
From among the known secondary cells and batteries, comparatively high energy densities are achieved by lithium-ion batteries in particular. In many cases, lithium-ion batteries contain a cell stack made up of a plurality of single cells. However, cells and batteries with high capacitance in particular are often constructed as winding cells which have strip- or ribbon-like electrodes in wound form (electrode windings). This is also the preferred design of capacitors.
Secondary cells and batteries and also capacitors with a winding structure usually have a wound composite comprising flat electrodes and separators in the sequence positive electrode/separator/negative electrode. In the case of secondary cells and batteries, the electrodes usually comprise metal current collectors and also electrochemically active components and electrochemically inactive components. Electrochemically active components (often also called active materials) for secondary lithium-ion batteries include all materials which can take up lithium ions and release them again. The prior art in this respect for the negative electrodes are, in particular, particles based on carbon such as graphitic carbon or non-graphitic carbon materials which are capable of intercalating lithium. Furthermore, it is also possible to use metallic and semimetallic materials which can be alloyed with lithium or composites of such materials with, for example, carbon-based materials. Lithium metal oxide compounds and lithium metal phosphate compounds such as LiCoO2 and LiFePO4 in particular can be used for positive electrodes. As electrochemically inactive components, mention may be made first and foremost of electrode binders and the mentioned current collectors. Electrons are supplied to or discharged from the electrodes via current collectors. Electrode binders ensure mechanical stability of the electrodes and that the particles comprising electrochemically active material contact one another and the current collector. Microporous plastic films in particular can be used as separators.
In the case of capacitors, the electrodes are usually electrically conductive substrates such as metals. A non-electrically conductive nonwoven, a porous plastic film (for example, polyethylene) or a non-electrically conductive porous ceramic layer (for example, aluminum oxide), for example, can serve as a separator.
To produce electrodes for secondary batteries and cells, pastes comprising the mentioned electrochemically active and inactive components are applied to the electrical collectors in the form of thin layers, dried and converted into the desired fitting shape. The layers are usually then rolled and pressed and possibly then combined with separators and counterelectrodes. However, problems may arise in the process, particularly when producing winding cells.
Current is discharged from electrode windings by collector lugs. When producing electrode windings, it is generally customary to provide the strip- or ribbon-like electrodes, which are to be wound, with a plurality of, preferably a large number of, collector lugs to subject the electrodes to loading in as uniform a manner as possible during charging and discharging. Accordingly, it is customary to provide collector lugs distributed over the entire length of an electrode strip to be wound. In the finished winding, these collector lugs usually protrude from the winding at the end face. In that case, collector lugs of the same polarity should be grouped, in particular such that they overlap in a stack-like arrangement so that they can be connected to one another as easily as possible, for example, by welding. So that a grouping of this kind is obtained and the collector lugs, for example, in spiral windings are not, for example, radially distributed in a more or less random manner, it is necessary to take into account that the radius of an electrode winding increases in the outward direction with each turn of the winding. If collector lugs are arranged at uniform distances on the longitudinal side or sides of an electrode strip, this leads to a radial offset of the collector lugs in the winding.
In practice, this problem is dealt with by the distances between adjacent collector lugs along the electrode strip to be wound being allowed to increase in the direction of that end of the strip on the outside of the winding. The required distances can be calculated in a very simple manner by taking into account the thickness of the electrodes. However, fluctuations in the thickness in the longitudinal direction occur particularly in the case of the electrodes of secondary batteries and cells within certain tolerances. Given a minimum thickness fluctuation of +/−1 μm, the resulting offset in a winding cell with more than 100 turns can already be several millimeters.